The present invention relates to apparatus and methods for replacing a core spray T-box and thermal sleeve forming part of a core spray system in a nuclear reactor and particularly relates to the replacement of the T-box and a portion of the thermal sleeve from within the reactor vessel.
In a typical boiling water nuclear reactor, a core spray system is provided to prevent excessive fuel clad temperature in the event of a loss of coolant accident by delivering cooling water to the core region of the reactor. A core spray system conventionally includes a core spray pump for pumping water through an external piping system in communication with a nozzle which penetrates the reactor vessel wall. Within the reactor vessel, the nozzle encompasses a thermal sleeve which terminates in a T-connection or T-box for delivery of the cooling water via an internal piping and nozzle system to the nuclear core. A pair of redundant systems are typically employed, each including a core spray nozzle having a nozzle bore penetrating the vessel wall and a nozzle safe end outside of the vessel wall for connection with the core spray piping external to the reactor vessel wall. On the interior side of the vessel wall, there is a T-box which is welded to the thermal sleeve within the nozzle bore and which sleeve is, in turn, welded to the nozzle safe end.
These welded joints are susceptible to cracking. In the event one or more of the welds crack, the integrity of the vessel wall is compromised. While it is possible to remove the safe end of the nozzle from a location outside the reactor vessel wall, substantial and unacceptable downtime and expense would be necessary to accomplish that task because external removal of the safe end invariably necessitates draining the reactor vessel. Consequently, there is a need to provide a simple, reliable connection between internal piping of the core spray system and the safe end of the nozzle which can be used to replace the T-box and thermal sleeve without removing and replacing the associated safe end thereby maintaining the integrity of the reactor vessel wall.
In accordance with a preferred embodiment of the present invention, there is provided apparatus and methods of mechanically attaching a replacement T-box and a thermal sleeve portion to an existing core spray nozzle safe end from within the reactor vessel. To accomplish the foregoing, and from within the reactor vessel wall, the existing, i.e., extant T-box and a portion of the thermal sleeve are first removed from the core spray nozzle. This can be accomplished by conventional underwater electric discharge machining (E.D.M.). Once the extant thermal sleeve portion is removed, the newly cut end of the thermal sleeve remnant is machined flat and perpendicular to the nozzle bore so that it will mate with a replacement assembly comprised of a combined T-box/thermal sleeve. An annular groove is also machined in the remnant end of the thermal sleeve, preferably along the inside diameter of the thermal sleeve remnant. The replacement T-box/thermal sleeve assembly is then inserted into the nozzle bore for mechanical securement with the thermal sleeve remnant, the invention being accomplished from within the reactor vessel.
A sealing washer, preferably a Belleville washer, is interposed between abutting ends of the replacement thermal sleeve and the thermal sleeve remnant such that in final securement, the replacement thermal sleeve and thermal sleeve remnant are placed in compression. To mechanically secure the replacement T-box/thermal sleeve assembly in the nozzle bore, a collet having a plurality of circumferentially spaced fingers terminating in radially directed flanges is secured, preferably by thrusting, to the assembly at an end of the collet opposite the flanges. Thus, the replacement thermal sleeve is secured in the nozzle in a manner such that the extent of the collet fingers can be adjustable relative to the replacement thermal sleeve. For example, the collet may be screwthreaded to and within the replacement thermal sleeve. When the replacement T-box/thermal sleeve and washer are inserted into the nozzle bore, the collet flanges ride over the end portion of the thermal sleeve remnant to engage in the groove. By adjusting the collet and replacement thermal sleeve relative to one another, i.e., by screwthreading, the securement between the replacement thermal sleeve and the thermal sleeve remnant places the Belleville washer under compression, as well as portions of the replacement thermal sleeve and thermal sleeve remnant.
To secure the fingers with their flanges in the groove, a retention sleeve is also screwthreaded to the replacement thermal sleeve and extends to overlie the fingers, preventing, in final securement, radial movement of the flanges of the fingers in a direction, e.g., radial inward direction, away from the groove. Once the collet finger flanges are locked in the groove, the retention sleeve is staked to the replacement thermal sleeve to prevent its release through relative rotation.
Additionally, a plurality of adjustable wedges are located between the replacement thermal sleeve and the nozzle bore adjacent the interior wall surface of the reactor vessel. By adjusting the wedges, the replacement thermal sleeve is axially aligned within the nozzle bore. The wedges also support the weight of the replacement T-box/thermal sleeve assembly from the vessel wall, thereby minimizing bending stresses on the collet fingers. An end cover is then threaded onto the open end of the T-box housing compressing a second Belleville seal ring between the cap and housing. An anti-rotation feature, such as staking, can be employed to finally secure the cover and the T-box to one another.
In a preferred embodiment according to the present invention, there is provided in a nuclear reactor having a reactor vessel wall and a nozzle penetrating the wall for delivering a fluid into the reactor, the nozzle including an end portion of a sleeve within the nozzle and having a groove, a connection with piping internal to the reactor vessel, comprising a thermal sleeve extending from within the reactor vessel wall and having an end adjoining the sleeve end portion within the nozzle, a generally cylindrical collet having a plurality of circumferentially spaced fingers with radially directed flanges adjacent one end thereof, the flanges engaging in the groove, the collet being connected to the thermal sleeve adjacent an end thereof opposite one collet end and a retention sleeve secured to the thermal sleeve and extending about the fingers to retain the flanges in the groove.
In a further preferred embodiment according to the present invention, there is provided in a nuclear reactor having a reactor vessel wall and a nozzle penetrating the wall for delivery of a fluid within the reactor, the nozzle including a nozzle bore and a nozzle safe end having a thermal sleeve within the nozzle bore coupled to a fitting adjacent the interior of the vessel wall, a method of replacing the fitting and a portion of the thermal sleeve from within the reactor vessel wall, comprising the steps of removing the fitting and a portion of the thermal sleeve from within the nozzle leaving a thermal sleeve remnant within the nozzle, the step of removing being accomplished from the interior side of the reactor vessel wall, forming a groove in the thermal sleeve remnant within the nozzle bore, inserting into the nozzle bore from within the reactor vessel a replacement thermal sleeve, engaging an end portion of the replacement thermal sleeve with an end portion of the thermal sleeve remnant and engaging circumferentially spaced fingers of a collet secured to the replacement thermal sleeve in the groove to couple the replacement thermal sleeve and the extant thermal sleeve remnant to one another.